Fix Mac builds when using llvm.
Fix Mac builds when using llvm.

supports_llvm needs to include is_mac.

Bug chromium:966543

Change-Id: Ieedf1bba3948de8c75d8bd07473fa2cb0a1e4299
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32032
Reviewed-by: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>
Presubmit-Ready: Sean Risser <2d04e98eeaa433fb55534399ab3ab2c06fdf7967@google.com>
Tested-by: Sean Risser <2d04e98eeaa433fb55534399ab3ab2c06fdf7967@google.com>
Kokoro-Presubmit: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader